1. Field of the Invention
The present invention relates to an exhaust gas recirculation control system for an internal combustion engine, more particularly a diesel engine.
2. Description of the Prior Art
Exhaust gas recirculation (E.G.R.) control systems are known as a system which suppresses combustion in an internal combustion engine to reduce NOx discharged from the engine. In order to improve the constituents of exhaust gases without imparing the driveability, the E.G.R. flow rate must be appropriately controlled depending upon varying operating condition of the engine.
One of the known E.G.R. control systems for a diesel engine is described in chapter EXHAUST EMISSION CONTROL SYSTEM of a printed publication entitled "1981 DATSUN 810 DIESEL SERVICE MANUAL" printed on March 1981. This known E.G.R. control system is hereinafter described in connection with FIGS. 1 and 2 wherein FIG. 1 is a schematic illustration of the known E.G.R. control system and FIG. 2 illustrates mode of operation of the E.G.R. control system.
Referring to FIG. 1, the reference numeral 1 designates an air cleaner, the reference numeral 2 an intake passage of a diesel engine 3, the reference numeral 4 an exhaust gas passage of the engine 3. The E.G.R. control system comprises an exhaust gas recirculation passage (E.G.R. passage) 5 connecting the exhaust gas passage 4 to the intake passage 2 at a portion thereof downstream of a throttle valve 6 disposed in the intake passage 2, and an E.G.R. control valve 7. The E.G.R. control valve 7 has a valve element 7a disposed within the E.G.R. passage 5.
The throttle valve 6 is operatively connected to a throttle diaphragm 8 and actuated such that when vacuum from a source of vacuum in the form of a vacuum pump 9 develops in a pressure chamber 8a of the throttle diaphragm 8, the throttle valve 6 is closed to a predetermined partially closed position wherein the throttle valve 6 is partially closed, whereas when the pressure chamber 8a is open to the atmosphere, the throttle valve 6 is fully opened. The source of vacuum may be in the form of a vacuum tank. The presseure chamber 8a of the throttle diaphragm 8 is connected selectively to the vacuum or the atmospheric pressure under the control of a three-way throttle solenoid 11 which is operable by a E.G.R. control unit 10. The E.G.R. control valve 7 includes a diaphragm 12 for actuating the valve element 7a. When the vacuum from the vacuum pump 9 develops in a pressure chamber 12a of the diaphragm 12, the valve element 7a is lifted and the E.G.R. control valve 7 is fully opened, whereas when the pressure chamber 12a is opened to the atmosphere, the E.G.R. control valve 7 is fully closed. The pressure chamber 12of the diaphragm 12 is connected selectively to the vacuum or the atmospheric pressure under the control of a second three-way solenoid 13 which is operable by the E.G.R. control unit 10.
The E.G.R. control unit 10 receives an information relating to the operating state of the engine from a load sensor in the form of a potentiometer 14, a revolution sensor 15 and a water temperature sensor 30. The potentiometer 14 is mounted to a control lever 32 of a fuel injection pump 34. The potentiometer 14 converts the opening angle of the control lever 32 controlled by an accelerator pedal 36 into a voltage signal. The revolution speed of the engine is measured the revolution sensor 15 which picks up the engine speed by counting the number of holes formed on a pulley 38 of the fuel injection pump 34. The engine temperature is measured by the water temperature sensor 30. The E.G.R. control unit 10 operates the solenoids 11 and 13 to control the E.G.R. flow rate in three modes I, II, and II as illustrated in FIG. 2. The mode I, "HIGH E.G.R.", is obtained in the combination of closed throttle valve 6 and open E.G.R. valve 7. In the mode I, a sufficient amount of E.G.R. gas is drawn into the intake passage 2 by the vacuum created downstream of the throttle valve 6. As will be understood from FIG. 2, the mode I is selected when the engine operates in a low load operating state wherein high E.G.R. is required.
The mode II, "LOW E.G.R." is attained by opening the throttle valve 6 with the E.G.R. valve 7 kept open. The E.G.R. control unit 10 selects the mode II when the load on the engine increases.
The mode III is "Zero-E.G.R." condition and is attained by closing the E.G.R. valve 7. The mode III is selected when the engine operates in heavy or high load state.
The mode of operation is tabulated as follows:
TABLE I ______________________________________ Mode Throttle valve 6 EGR valve 7 EGR flow rate ______________________________________ I Closed Open High II Open Open Low III Open Closed Zero ______________________________________
The solenoid 11 is designated to connect the pressure chamber 8a of the throttle diaghragm 8 only to the vacuum pump 9 when it is off or deenergized. When it is on or energized, the pressure chamber 8a of the throttle diaphragm 8 is connected only to the air cleaner 1. The solenoid 13 is the same construction as the solenoid 11 and connects the pressure chamber 12a of the diaphragm 12 only to the vacuum pump 9 when it is off or deenergized. When it is on or energized, the pressure chamber 12a is connected only to the air cleaner 1. The relationship of the state of each of the solenoids 11 and 13 with the three modes is as follows:
TABLE II ______________________________________ Mode Sol. 11 Sol. 13 ______________________________________ I Off Off II On Off III On Off ______________________________________
With the known E.G.R. control system, when the accelerator pedal 36 is released to change the load on the engine from high load state into low load state, the throttle valve 6 is closed and the E.G.R. control valve 7 is opened to accomplish the mode I (see TABLE I). A problem encountered in this known E.G.R. control system is that composition of combustible charge is deteriorated upon depressing the accelerator pedal 36 for reacceleration immediately after the accelerator pedal 36 has been released for a moment, which occurs, for example, upon completion of a gear shifting operation during acceleration of an automotive vehicle from the standstill, resulting in the production of smoke and an increase in HC emission. This is attributed to the fact that with the diaphragms 8 and 12, the opening of the throttle valve 6 and closing of the E.G.R. control valve 7 cannot be effected as quickly as the quantity of fuel is increased, resulting in momentary shortage of intake air which creates rich air fuel mixture.